The distal airways of the lung are lined with an acellular layer of material which appears vital in the maintenance of normal pulmonary function. The composition and origins of the acellular lining are being investigated. Current investigations have been concerned with 1) the biosynthesis and secretion of pulmonary surfactant, 2) the acellular lining as an enzymatically active medium, and 3) the protein composition of the lining. BIBLIOGRAPHIC REFERENCES: Saru, S., DiAugustine, R. P., and Lynn, W. S.: Lipids in the lung lavage of patients with alveolar proteinosis and in rabbit lung lamellar organelles. Am. Rev. Resp. Dis. 114: 177-185, 1976. Hook, G. E.R. and DiAugustine, R. P.: Secretory cells of peripheral pulmonary epithelium as targets for toxic agents. Env. Hlth. Perspect. 16: 147-156, 1976.